Need User Guide for ERA security front door lock

We recently had some building work done on our house and asked the builder to replace the elderly falling to bits yale lock on our front door with something more secure. The builder is a great guy but if he has one fault it is that he throws away the instructions and packaging for everything he installs as soon as he unpacks it. Knowing this I usually manage to get there first and save the instructions, warranties etc but did not manage it on this occasion.

So we have a very smart new ERA British Standard Double Locking Nightlatch but it is proving difficult to use. Basically the problem is that there is a two part latch, the main latch which you turn with the internal handle when indoors to open it and and a separate smaller latch which you can set to stay rigid (using a small black button at the top of the lock) if you want to leave the door on the latch. This smaller latch sticks in the on position. I think we need to adjust it and some instructions would be useful.

I phoned the builder and he said he bought it from his local DIY store which is not near us, so I can't go and read up on it. Nor are there any instructions on the ERA website. Does anyone know any major chains who stock so I can go and read it up in the store!